1. Field of the Invention
The present invention relates to the field of rivets, and more particularly to a method of manufacture of aluminum solid rivets for aircraft and other high performance and high endurance applications.
2. Prior Art
Of particular interest to the present invention are rivets having a tapered or conical region extending from the shank of the rivet, usually integrally joining the shank to a substantially cylindrical rivet head. Rivets of the general type described are used in large quantities in such applications as in wet wing structures. In these applications, the rivet heads expand radially during setting of the rivets so that the periphery of the rivet heads seal with respect to corresponding countersunk holes in one of the skin members to be joined.
Solid rivets, whether for aircraft use as described above, or for other uses, are generally fabricated in large numbers starting with a wire, rod or bar of material of substantially the same diameter as the desired shank of the finished rivet. In fabrication, the rod is cut off, the end of the rod is inserted into the die defining the rivet, and then typically given an initial upset, followed by a final blow to form the head and tapered region between the head and shank of the rivet.
In modern aircraft applications, solid rivets may be subjected to relatively high repetitive loads due to repeated pressurization and depressurization of the cabin, the flexing of structures due to turbulence, takeoffs and landings, engine and other equipment vibration, etc. Further, modern jet aircraft tend to have a high usage factor and are generally maintainable almost indefinitely, tending to bring out some undesired characteristics of components such as solid rivets, heretofore considered relatively indestructible.
In particular, it has been noted that after long service, the heads, or portions of the heads, of some solid rivets will simply fall off, requiring replacement of the rivets. Inspection of the end of the remaining rivet shank indicates that such failures are frequently due to fatigue and/or stress corrosion cracking at the juncture between the shank and the tapered region. (Stress corrosion is an accelerated corrosion caused by substantial stresses on a part, a material under stress normally corroding substantially faster than the same material in the same environment but not under stress. Fatigue, on the other hand, is caused by the cycling of stresses, eventually causing a surface crack to develop and then progress through the part until the same fails.)
The prior art method of fabricating solid rivets, and particularly aluminum aircraft solid rivets, for installation into a countersunk hole in the work pieces as described above, is illustrated with reference to FIGS. 1 through 4. In particular, FIG. 1 is a cross-section of a typical prior art die 20 defining a cylindrical rivet head region 22, a shank region 24 and a tapered region 26 connecting the shank region 24 with the head region 22. This die is used in a header machine, typically a two blow header, which automatically feeds and shears a length of wire, bar or rod 28 and places same into the forming die, as shown in FIG. 2. As may be seen therein, the resulting rivet blank 28 is of a diameter approximately equal to the rivet shank diameter as defined by region 24 of the die 20. On the first header blow, head 30 of the rivet will be partially formed as shown in FIG. 3, and then as shown in FIG. 4, a second blow will finish the rivet, the rivet then being expelled from the die by an ejection pin inserted through opening 32 at the shank end of the die.
The foregoing method of manufacturing rivets is fast and inexpensive, and is capable of providing rivets of good dimensional accuracy. However, as more and more is expected of such rivets, it would be desirable to reduce or eliminate the potential for fatigue or stress corrosion cracking resulting from prolonged use. Also in the case of aircraft rivets used in the fabrication of wet wing structures (aircraft wings wherein the wing skin also forms an exterior wall of a fuel tank as mentioned above), longitudinally oriented marks on the surface of rivets can provide fuel leak paths in the set rivet. Consequently, either the leaking rivets must be drilled out and replaced, or extra and expensive processing must be undertaken during the rivet manufacture, such as first fabricating the rivets oversize, and then profile grinding the same to remove the surface imperfections and to provide a smooth surface that will set without leaking. Alternatively, the rivet wire used in the fabrication of solid rivets can be shaved prior to use in forming rivets to remove any longitudinal surface imperfections caused by the drawing of the wire, such as a double shave by running the raw material through diamond dies. Still, the occurrence of leakers is not eliminated, and as such, shaving has heretofore had limited success.